This proposal seeks to investigate a novel biologic mechanism behind the pathogenesis of craniosynostosis. Craniosynostosis is a debilitating clinical condition characterized by the premature fusion of cranial sutures. This premature fusion results in an aberrant craniofacial shape and increased intracranial pressure. The prevalence of craniosynostosis is high, at approximately 1 in 2500 live births, and current treatment is limited to genetic counseling, surgery, orthodontics, social and medical support. It has been known for over a decade that craniosynostosis is associated with activating mutations in fibroblast growth factor receptors (FGFR's), yet the molecular mechanism by which these mutations lead to the disease phenotype is unknown. One mechanism by which activating mutations in FGF receptors potentially mediate abnormal tissue mineralization may involve ectonucleotide pyrophosphatase/phosphodiesterase-1 (PC-1), the primary osteoblastic generator of pyrophosphate, and tissue non-specific alkaline phosphatase (TNAP), whose enzymatic activity is essential for the deposition and growth of hydroxyapatite crystals. Evidence in support of this mechanism is provided by studies from our laboratory and those of others showing that: (1) FGF signaling induces PC-1 and inhibits TNAP expression in osteoblastic cells and calvarial tissues;(2) PC-1 and TNAP regulate bone mineralization and soft tissue calcification via the generation of pyrophosphate and hydrolysis of pyrophosphate to phosphate, respectively;and (3) craniosynostosis occurs in humans with inactivating mutations in TNAP. Together these findings suggest a model for FGFR-associated craniosynostosis in which activating mutations in FGF receptors enhance PC-1 and inhibit TNAP expression, leading to increased ratios of inorganic pyrophosphate to phosphate, with diminished calvarial bone mineralization and enhanced calcification of non-mineralized tissues, including the cranial sutures. While our preliminary studies provide strong evidence for this model, the significance of pyrophosphate generation and hydrolysis in the pathogenesis of craniosynostosis has yet to be definitively established. Accordingly, we will investigate PC-1 and TNAP as critical mediators of the effects of FGF signaling on calvarial bone mineralization and craniosynostosis. Results from the proposed research will help to elucidate the molecular pathogenesis of craniosynostosis and provide strong rationale for the development of biologic therapeutics for the treatment of patients with this debilitating condition. PUBLIC HEALTH RELEVANCE: Craniosynostosis is a debilitating clinical condition characterized by the premature fusion of cranial sutures. The prevalence of craniosynostosis is high, at approximately 1 in 2500 live births, and current treatment is limited to genetic counseling, orthodontics, surgery, medical and social support. Results of the proposed research will help to elucidate the molecular pathogenesis of craniosynostosis, thus leading us closer to the development of interceptive or adjunctive biologic treatments for patients with this medically and socially challenging condition.